1. Field of the Invention
The present invention generally relates to a magnetic refrigerant material. More particularly, the present invention relates to a magnetic refrigerant material that exhibits a sufficiently great magnetocaloric effect near or at room temperature, and also relates to a regenerator and a magnetic refrigerator that use the magnetic refrigerant material.
2. Description of the Related Art
Conventional refrigeration technology has often utilized the adiabatic expansion or the Joules-Thomson effect of a gas. However, such gas compression technology has some drawbacks. Firstly, a chlorofluorocarbon (CFC) gas, a typical refrigerant working material used most commonly in this technology, damages the ozone layer, which makes this material environmentally hazardous. Secondly, the gas compression technology always results in low efficiency, thus constituting a huge obstacle to the desired energy saving. To solve these problems, a method that takes advantage of entropy change accompanied by the magnetic phase transition (which is also termed xe2x80x9cmagnetic transformationxe2x80x9d) of a solid has been researched as a high-efficiency refrigeration technique. In the magnetic refrigeration technique, cooling is effected by using a change in temperature resulting from the entropy change of a magnetic material. More specifically, a magnetic material used in this method alternates between a low magnetic entropy state with a high degree of magnetic orientation, which is created by applying a magnetic field to the magnetic material near its Curie temperature, and a high magnetic entropy state with a low degree of magnetic orientation (e.g., randomly oriented state), which is created by removing the magnetic field from the magnetic material. A property like this is called xe2x80x9cmagnetocaloric effectxe2x80x9d and a magnetic refrigerator, which uses a material exhibiting the magnetocaloric effect (which will be herein referred to as a xe2x80x9cmagnetocaloric materialxe2x80x9d) as its magnetic refrigerant material or regenerative material, has been researched and developed vigorously.
A known magnetocaloric material (e.g., metallic gadolinium (Gd)) exhibits a second order phase transition. Such a material exhibits the magnetocaloric effect in a relatively broad temperature range, but the magnitude of the magnetocaloric effect is relatively small. Accordingly, to achieve cooling power at a practical level, a high magnetic field as strong as 5 T (tesla) or more, which can be generated only by a superconducting magnet, for example, must be applied to the magnetic material. Therefore, a large quantity of energy is consumed to apply that strong magnetic field. That is to say, the desired reduction in energy consumption, which should be one of the major advantages of the magnetic refrigeration technique, is not realizable by this magnetic material alone.
On the other hand, a magnetocaloric material, which exhibits a first order transition from a ferromagnetic phase into a paramagnetic phase at its Curie temperature, exhibits the magnetocaloric effect in a relatively narrow temperature range. However, the magnitude of the magnetocaloric effect is relatively great. Accordingly, such a material is the object of much attention because it is highly likely that the material can be used in a regenerator or a magnetic refrigerator that operates on the application of a magnetic field generated by a permanent magnet. See, for example, Materia, Vol. 39, No. 11, pp. 909-915, November 2000, xe2x80x9cMagnetocaloric Effect of Compounds Showing First Order Phase Transition (A Research of High-Efficiency Magnetic Refrigeration)xe2x80x9d, Hirofumi Wada et al. More particularly, it was recently discovered that an intermetallic compound Gd5 (SixGe1xe2x88x92x)4 (where xxe2x89xa60.5) shows a first order magnetic phase transition at a temperature that is near room temperature and the compound is now believed to potentially provide a regenerator and a magnetic refrigerator that can operate at room temperature. See, for example, V. K. Pecharsky et al., Appl. Phys. Lett., 70, pp. 3299-3301 (1997).
However, even if the intermetallic compound is used, it is still necessary to apply a strong magnetic field to realize cooling power at a practical level. Thus, there is a growing demand for a material that exhibits an even greater magnetocaloric effect.
In order to overcome the problems described above, preferred embodiments of the present invention provide (1) a magnetic refrigerant material that exhibits sufficiently great magnetocaloric effect at or near room temperature and (2) a regenerator and a magnetic refrigerator using such a magnetic refrigerant material.
A magnetic refrigerant material according to a preferred embodiment of the present invention preferably has an NiAs type hexagonal structure in a ferromagnetic phase. The material includes a first element Mn, a second element As and a third element to substitute for a portion of the second element. In addition, the magnetic refrigerant material exhibits a magnetic phase transition in a temperature range from about 230 K to less than about 318 K.
In one preferred embodiment of the present invention, the third element is preferably Sb. Optionally, the magnetic refrigerant material may also include a fourth element.
Specifically, the magnetic refrigerant material is preferably represented by the general formula MnAs1xe2x88x92xSbx (where 0 less than xxe2x89xa60.25) and preferably exhibits the magnetic phase transition into the ferromagnetic phase substantially without undergoing a structural transformation when a magnetic field is applied to the material in a paramagnetic phase. More particularly, x is preferably equal to or greater than about 0.015, more preferably equal to or greater than about 0.05.
In this particular preferred embodiment, the material may exhibit the magnetic phase transition upon the application of a magnetic field of about 4 T (tesla) or less.
A regenerator according to another preferred embodiment of the present invention preferably includes first and second regenerative beds, each including the magnetic refrigerant material according to any of the preferred embodiments of the present invention described above, and a mechanism for applying mutually different magnetic fields to the first and second regenerative beds.
In one preferred embodiment of the present invention, each of the first and second regenerative beds may include a plurality of magnetic refrigerant materials that exhibit the magnetic phase transition at respectively different temperatures.
Specifically, the magnetic refrigerant materials may form multiple layers that are stacked one upon another.
In another preferred embodiment of the present invention, each of the first and second regenerative beds may include the magnetic refrigerant material and a binder. In that case, the binder may preferably be Al, Cu or Ti. Alternatively, the binder may also be a mixture or alloy that includes two or more elements selected from the group consisting of Al, Cu and Ti, or other suitable material.
In still another preferred embodiment, the mechanism for applying the magnetic fields may include a magnetic circuit including a permanent magnet.
More particularly, the magnetic circuit may variably control the strengths of the magnetic fields to be applied to the first and second regenerative beds.
Alternatively, the regenerator may further include a mechanism for shuttling the first and second regenerative beds back and forth between a first position, which is inside the magnetic field created by the permanent magnet, and a second position, which is outside of the magnetic field, thereby applying the mutually different magnetic fields to the first and second regenerative beds.
A regenerator according to still another preferred embodiment of the present invention preferably includes a magnetic circuit for variably controlling the strength of a magnetic field generated therefrom inside a cylindrical space, and a regenerative bed, which is disposed and fixed inside the cylindrical space and includes the magnetic refrigerant material according to any of the preferred embodiments of the present invention described above.
A magnetic refrigerator according to still another preferred embodiment of the present invention preferably includes the regenerator according to any of the preferred embodiments of the present invention described above, and a cold-side heat exchanger and a hot-side heat exchanger that are thermally coupled to the regenerator.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.